I've been asked to spec out and install a PA system in 4 halls in a Church. There huge, not Westminster Cathedral but its the biggest church I've ever been in.

The main hall is 160 ft by 150 ft, the ceiling height is 180ft. Its a morbid damp stone building with fine acoustics for a bunch of choir boys I'm sure. Yet I've never done a church with echo for days.

They have no budget to speak of and want massive monoliths with drive units stacked to heaven. My plan is to install four (12 inch 4 ohm) drive units per side to just flood the place with sound from 40Hz/2.5Khz, maybe some horn drivers and strap a Cloud type PA amplifier to them. All made from 18mm ply. There will be no cross overs on the 12 inch drive units. I'll wire them into dual series and then parallel loads per side to maintain 4 ohms for the amplifiers. This must be as cheap as possible, I know they probably won't pay for all the hardware never mind my time and labor. This is a real charity case and a personal challenge for me.

Any suggestion on the best bang for the buck are welcome as I've never done PA systems on a shoe string budget.

no budget ha?
in that case I highly recommend you go ahead and buy remaining AURA 3" fullrange drivers
they deliver wonderfull midrange, I know, I used them in many projects
and you get them for $1 each!
they are 16 ohm, so just slap 4 in paralell and you get 4 ohm, that you make as many 4 ohm sets you need to get coverage
you will not regret it!The Madisound Speaker Store

Require the women to wear hats, and the men to wear suits. Pad the seating. Carpet the aisles.

Then add some pitch shifting to that natural reverb for that darth-vader fear-of-God sermon... Nobody will understand a word, but they'll all suddenly become believers. "Death Comes Unexpectedly..." Excuse me if I push the list rules, staying on the acoustic attributes and not getting into the realm of the supernatural.

i'd like to know if they have an existing pa and what it consists of i.e. mixer/amp,music sources,speakers 70 v distributed or...
in reading your initial post it sounds like your determined to make this rig stereo but given your architectural acoustic situation stereo imaging and original sources like an open mic may not mesh well unless properly implemented.
central clusters (speakers) hung over the source area generally yield the best overall coverage while minimizing comb and reflec room effects.if i am picturing the room as stated it's as wide as it's deep with a high centrally vaulted ceiling correct?
then two vertical (column) arrays of four 12's in a coincident pair would be the cat's meow.

Adason's suggestion for the Aura's is a good one. Used in long line array's would help beaming in the vertical plane so you can throw less energy on the floor and into the ceiling, reducing echo. Best way to achieve some modicum of intellibility at reasonable cost. Forget modern music. The acoustics of a large church belongs to a certain kind of music, in effect, choir/organ mucic and the acoustics of churches grew up together. Modern music is just too fast and the whole place will just muddy up with reflections, whatever PA you put in.

actually that will be certain death, vacuphile, since it will throw way too much energy into the side walls, which are hard stone... (I assume). Ceiling echo is likely fairly easy to discern as a separate sound compared to side echo which will arrive much closer in time to the original signal, more so the farther back you get...

You might want to study those phased arrays that they are using in stores now to put sound directly in front of the screens, etc... that might be a solution in such a situation...